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From: cem@intelca.UUCP (Chuck McManis)
Newsgroups: net.analog
Subject: Re: Re: Any 555 mavens or PUT weenies out there?
Message-ID: <557@intelca.UUCP>
Date: Wed, 17-Apr-85 11:15:38 EST
Article-I.D.: intelca.557
Posted: Wed Apr 17 11:15:38 1985
Date-Received: Fri, 19-Apr-85 01:41:36 EST
References: <469@umd5.UUCP> <396@psivax.UUCP>
Organization: Intel, Santa Clara, Ca.
Lines: 26

> In article <469@umd5.UUCP> zben@umd5.UUCP (Ben Cranston) writes:
> >The situation is that I want to build a bitbanger serial interface for my
    ... Message about problem goes here ...
Followd by answer : 
> 
> You can run an RS232 with the voltages +5 and -5.  You already have the +5.
> In order to make the -5 you need to get an Intersil 7660 (negative voltage
> generator).  I have heard that this part is available at such places as
> Radio Shack.  This chip and two capacitors is all that is required to generate
> the -5 voltage.
> -- 

I tried this method once and found that the Intersil part couldn't supply
enough current to run the data line, much less the handshake lines over 
any length of cable. The voltage went from -4.8V to -2V and out of the
allowable +/- 3V specs for RS-232C. However, if you first use your handy
555 voltage doubling circuit and feed the 7660 with 9.9V which is inverted
to -9.7 volts which only degrades to -5.5 volts when running all of the 
handshake lines too. It works quite nicely.

--Chuck
-- 
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